1. Field
Embodiments of the invention relate to electronic devices, and more particularly, in one or more embodiments, to a phase locked loop system.
2. Description of the Related Technology
Phase locked loops (PLL) are useful building blocks for electronic circuits. In a PLL, the output signal should be phase and frequency locked to an input reference signal. A PLL that has a frequency divider inserted in the feedback loop can be used as an integer-N frequency synthesizer. For an integer-N frequency synthesizer, the signal at the phase detector negative input is phase and frequency locked to the reference. At steady state, the output frequency and phase is N times the reference frequency and phase. Output frequencies can be synthesized in steps of the reference frequency by setting the value of N.
In an integer-N frequency synthesizer, at steady state, there should be exactly N periods of the output for every period of the reference. Therefore a rising edge of the output at a period Nis in phase with a rising edge of the reference.
For finer adjustment of an output frequency and improved performance of a PLL, a fractional-N synthesizer may be used. In a fractional-N synthesizer, the divider in the feedback path has an integer part and a fractional part and the output frequency step resolution can be a fraction of the reference frequency. The output may be described according to equation 1.FOUT=(N+F/M)×FREF   Eq. 1
The fractional part may be generated with a digital interpolator. An output of the digital interpolator includes a sequence of integer values with an average value given by F/M, in which F is an input value and M is the modulus. The interpolator may be, for instance, a single accumulator with an overflow bit set as the output or may be a higher order sigma-delta modulator.
Varying the output frequency in a fractional-N synthesizer is useful in applications such as wireless systems (e.g., cellular phones) where two or more channels at the same frequency should have different phases to reduce interference. Applications of a fractional-N synthesizer with programmable output phase also include phased array radar systems where RF waves are transmitted at the same frequency but at different phases to form constructive and destructive interference (e.g., beam forming). Beam forming applications may also include focusing RF waves generated by broadcast stations, such as cellular phone base stations.